(a) Field of the Invention
The invention relates to a waveguide type light receiving element, and more particularly, to a waveguide type light receiving element which exhibits an excellent distortion characteristic over a broad band range and which is optimal for use in an optical communication system, such as an AM-FDM system, by attenuating incident light based on the intensity of the incident light to a given level.
(b) Description of the Related Art
In an optical communication system including the optical CATV system, amplitude modulation-frequency multiplex is effected by using AM(Amplitude Modulation)-FDM(Frequency Division Multiplex) method.
In the prior art, such an optical communication system employed a surface input type pin photodiode as shown in FIG. 11.
In the surface input type pin photodiode, an absorption layer, which is formed by InGaAs layer, is formed to have a large thickness, which is normally about 3 .mu.m, to achieve a satisfactory light absorption efficiency. Since the carriers pass through the thick absorption layer to increase the transit time thereof and to retard the response, and also since an extensive light receiving area is required to achieve a sufficient sensitivity, the capacitance tends to be increased to thereby limit the operable band range to below 2 GHz.
However, the AM-FDM system mentioned above requires a light receiving element which is operable over a broad band range while exhibiting a low distortion characteristic.
Accordingly, attention is recently directed to a waveguide type pin photodiode as illustrated in FIG. 12, wherein FIG. 12(a) shows a schematic perspective view of the waveguide type pin photodiode and FIG. 12(b) shows the layer structure thereof together with the refractive indices of individual layers.
First, with the light receiving element, an incident light is received by a light incident surface which is formed on an end surface of the waveguide, and is absorbed during its propagation through the waveguide. Accordingly, the light absorption layer is formed to be as thin as about 0.6 mm. In addition, the light density of the incident light can be increased, which allows the area of the light receiving region to be reduced, thereby reducing the capacitance. Second, the light absorption layer, which is formed by non-doped InGaAs layer, is sandwiched at its upper and lower surfaces of the light absorption layer between a pair of p-InGaAs layer and n-InGaAsP layer which are heavily doped with impurities. Accordingly, the operable band is determined by the carrier transit time in the lightly doped thin InGaAs layer.
That is, the waveguide type light receiving element is operable in a ultra-high speed over a broad band range as high as 50 GHz or above.
However, although one of the important characteristics which are required of a light receiving element used in an analog optical transmission system is a wide dynamic range response, the conventional pin photodiodes of waveguide type do not exhibit a satisfactory wide dynamic range response. Specifically, a light receiving element which is constructed as having a sufficient sensitivity to a low signal amplitude loses its linearity in the relationship between the light signal and photo-current converted therefrom when it receives a large amplitude light signal having a higher intensity, thereby degrading its distortion characteristic. Accordingly, such a light receiving element suffers from a problem of limitation in the intensity of the input signal light.
In light of the foregoing problem, it is an object of the invention to provide a waveguide type light receiving element which exhibits a wide dynamic range response.